The field of smart materials and intelligent structures has been gradually developing over the past few decades, increasingly enabled by technological advances in the areas of sensors, engineering materials, and actuators. The basis of many actuator and sensor technologies has increasingly been found in emerging “active materials.” Active materials, as a category, are materials that change their shape in response to an external control stimulus, typically a field, such as a thermal, magnetic, or electric field, but also radiation (light) or a changing chemical environment. Materials in this broad category include several classes, often delineated by the stimulus and material type: shape memory alloys, (SMAs), shape memory polymers (SMPs), piezoelectric ceramics, magnetostrictives, and electroactive polymers. Within each of these classes, there are many materials; e.g., within electroactive polymers alone there are a wide variety of low- and high-voltage-activated materials with widely-varying properties, such as ionic-polymer metal composites, conductive polymers, gels, and others.
Expandable structures have been applied to a myriad of uses. For example, when transporting an object into outer space, it may be advantageous to have the object compacted during the trip so that it may be deployed into its full form once in outer space. Attempting to fulfill such a need, non-active cellular metal or composite structures, such as those made by Hexcel Corporation, located at 281 Tresser Blvd., Stamford, Conn. 06901, have been used in space, aerospace and in automotive applications.
U.S. Pat. No. 5,164,129, issued to Douglas, discloses a self-deploying structural element that can be used as the building block for deployable structures. However, the Douglas invention is limited to forming rigid tubular structures and does not provide for other shapes.
Another deployable structure is described in U.S. patent application Ser. No. 09/771,455, issued to Sokolowski. The Sokolowski application describes a shape memory polymer polyurethane foam that can be compacted, and when heated to a prescribed temperature will recover its original shape and volume. Although the Sokolowski invention provides a reformable material, it is limited to foam applications.
U.S. Pat. No. 4,044,358, issued to Manning et al., describes a self-erectable structure that uses active materials to erect the structure to a predefined shape for radar attenuation. Although the Manning patent discloses using active materials, the Manning invention erects itself by using spring-like elements that actuate when physically unconstrained to deploy the structure and not necessarily by using a material with a controllable recoverable strain. In other words, the structure described in the Manning patent is not active in the sense that when an external force permanently deforms the “active” members, the members are no longer “active,” i.e., they cannot be actuated and to recover a memorized state.
Thus, a continuing need exists for a structure using tailorable, active materials that can be actuated to recover a memorized state.